waves Flashcards

Question

partial plane polarisation

Answer 1

When light is reflected from a reflective surface e.g. the surface of water or a wet road, it undergoes partial plane polarisation This means if the surface is horizontal, a proportion of the reflected light will oscillate more in the horizontal plane than the vertical plane

Answer 2

Polaroid cameras work in the same way as polaroid sunglasses They are very useful for capturing intensified colour and reducing glare on particularly bright sunny days Vertically polarised filters also enable photographers to take photos of objects underwater This is because the light reflected on the surface of the water is partially polarised in the horizontal plane This glare is eliminated by the Vertically polarising lens However, the light from the underwater object is refracted by the surface of the water, not reflected, so it is not plane-polarised Therefore, the light from the underwater object is more intense than the glare and shows up much more brightly in the photo

Answer 3

Radio and television services sent as radio waves are almost always plane polarised, so a receiving antenna needs to be aligned with the plane of the wave from the transmitter to obtain the strongest signal. The particular orientation of a receiving antenna will depend on the transmitter it is pointing towards and the polarity of the services being broadcast

Answer 4

downwards (Hint: draw the wave moving forward a little)

Answer 5

To the left Particles in rarefactions travel opposite to the direction of energy transfer

Answer 6

To the right Particles in compressions travel in the direction of energy transfer

Answer 7

The principle of superposition says that when two or more waves cross, the resultant displacement equals the vector sum of the individual displacements. You can use the same idea in reverse — a complex wave can be separated out mathematically into several simple sine waves of various sizes.

Answer 8

When two waves meet, if their displacements are in the same direction, the displacements combine to give a bigger displacement. A crest plus a crest gives a bigger crest. A trough plus a trough gives a bigger trough.

Answer 9

If a wave with a positive displacement (crest) meets a wave with a negative displacement (trough), they will undergo destructive interference and cancel each other out. The displacement of the combined wave is found by adding the displacements of the two waves

Answer 10

A stationary (standing) wave is the superposition of two overlapping progressive waves with the same frequency and amplitude, moving in opposite directions. Unlike progressive waves, no energy is transmitted by a stationary wave.

Answer 11

A stationary wave is made up nodes and antinodes At points where the two waves are in antiphase a node (0 amplitude) forms. Nodes are regions where there is no vibration. At points where the waves are in phase an antinode (maximum amplitude) forms. The nodes and antinodes do not move along the string. Nodes are fixed and antinodes only move in the vertical direction. No energy is transferred along the wave, it just oscillates between the kinetic and potential stores of the medium

Answer 12

* Vibrations caused by stationary waves on a stretched string produce sound * This can be demonstrated by a length of string under tension fixed at one end and vibrations made by an oscillator: * The wave generated by the oscillator is reflected back and forth. * At specific frequencies, known as resonant frequencies, a whole number of half wavelengths will fit on the length of the string * As the resonant frequencies of the oscillator are achieved, stationary waves form

Answer 13

There are 2 overlapping waves - same frequency, wavelength, speed and amplitude but opposite direction of travel - one is the reflection of another off the fixed ends. The waves superpose. Nodes (0 amplitude) form when phase difference = 0 radians anti Nodes (max amplitude) form when phase difference = pi radians no energy transfer along the string

Answer 14

Points on a stationary wave can only ever be in phase or in antiphase with one another

Answer 15

A stationary wave is only formed at a resonant frequency (when an exact number of half wavelengths fits on the string). There are some special names for each resonant frequency.

Answer 16

As the frequency of the vibration generator increases, different stationary waves are set up.. The first stationary wave will occur on a string of length L when the generator is vibrating at the fundamental frequency, f0, of the string.

Answer 17

You can set up a stationary wave by reflecting a microwave beam at a metal plate. The superposition of the wave and its reflection produces a stationary wave. You can find the nodes and antinodes by moving the probe between the transmitter and reflecting plate. The meter or loudspeaker receives no signal at the nodes and maximum signal at the antinodes.

Answer 18

Powder in a tube of air can show stationary sound waves. A loudspeaker produces stationary sound waves in the glass tube. The powder laid along the bottom of the tube is shaken away from the antinodes but left undisturbed at the nodes. The distance d between each pile of powder (node) is λ / 2, so the speed of sound c = fλ is equal to c = f x 2d = 2df. So the speed of sound can be calculated by measuring d and knowing the frequency of the signal generator.

Answer 19

A sound wave can be reflected by an open or a closed end of a pipe Closed ends form nodes, while open ends form antinodes

Answer 20

The spreading out of waves when they pass through a gap or by an edge It occurs when the size of the aperture or obstacle is of the same order of magnitude as the wavelength of the incident wave. * The longer the wavelength the greater the diffraction. * The smaller the aperture the greater the diffraction

Answer 21

a particular example of superposition when both waves have a CONSTANT PHASE DIFFERENCE and SAME FREQUENCY these waves are said to be coherent

Answer 22

For coherent sources, for any point where the path difference between the waves is a whole number of wavelengths, the phase difference between the waves will be 0 or an integer multiple of 2π, so the waves arrive in phase and superpose to give a large amplitude wave - a maximum

Answer 23

For coherent sources, for any point where the path difference between the waves is (n+0.5)λ where n is an integer i.e. 0.5λ, 1.5λ, 2.5λ, the phase difference between the waves will be an odd multiple of π, so the waves arrive in antiphase and superpose to give a low amplitude wave – a minimum. If the two waves have the same amplitude at that point in space the amplitude of the resultant waveform at that point will be zero.

Answer 24

The difference in distance travelled by two waves from their sources to the point where they meet.

Answer 25

Lasers are the ideal piece of equipment to analyse diffraction and intensity patterns because they form light that is: Coherent (have a constant phase difference and frequency) Monochromatic (have the same wavelength)

Answer 26

Lasers produce a very high-energy beam of light This intense beam can cause permanent eye damage or even blindness

Answer 27

It's important to use lasers safely and follow the guidelines: * Do not look into laser * Do not point the laser at others * Avoid (accidental) reflections (e.g. by keeping experiement well below eye level) * Display 'laser on' warning light outside room

Answer 28

Two speakers will act as coherent sources if they are driven from the same signal generator (they will emit in phase at the same frequency). The diffraction patterns from the speakers overlap and the waves superpose. At positions where the path difference is an integer number of wavelengths, as in the diagram, the two waves will be in phase and maxima will be observed. At positions where the path difference is an odd multiple of half-wavelengths, the two waves will be in antiphase and minima will be observed.

Answer 29

Coherent microwave sources can be produced by driving two microwave emitters from the same source or by placing a pair of parallel slits (double slits) in front of one emitter. Microwave emitters produce polarised waves, so it is important to consider the orientation of the sources and the detector.

Answer 30

In the case that the slit is an infinitesimally thin point source of light, the interference pattern consists of evenly spaced light and dark fringes of equal intensity - faintly shown in diagram. In the normal case that the slit is not an infinitesimally thin point source of light and has a measurable width, the interference pattern consists of evenly spaced light and dark fringes which decrease in intensity as you go further from the centre - highlighted in diagram

Answer 31

same as monochromatic light BUT White central maximum, uniform width bright coloured side fringes with blue closest to centre and red furthest away.

Answer 32

central magenta maxima - path difference of blue and red light is both 0 at this point blue light has a shorter wavelngth thus a shorter path difference and will form as a maxima cloaser to the centre than red light each blue maxima is one blue wavlength away from each other - same for red eventually they would meet again forming another magenta

Answer 33

* used sunlight * passed light through a filter so it was monochromatic * a single slit was used to diffract the light to illuminate two slits * as it is different paths of the same wave passing through both slits they act as a coherent source (constant phase diference)

Answer 34

There is a parallel line approximation - X1 and x2 are parallel

Answer 35

Isaac Newton (1672) - corpuscles Christiaan Huygens (1678) - Wave Theory of Light Thomas Young (1801) - double-slit experiment James Clerk Maxwell (1862) - EM waves and the wave equation. Albert Einstein (1905) - photoelectric effect Later the scientific community came to understand that light behaves both like a wave and a particle. This is known as wave-particle duality

Answer 36

Newton proposed that visible light is a stream of microscopic particles called corpuscles However, these corpuscles could not explain interference or diffraction effects, therefore, the view of light as a wave was adopted instead

Answer 37

Huygens came up with the original Wave Theory of Light to explain the phenomena of diffraction and refraction This theory describes light as a series of wavefronts on which every point is a source of waves that spread out and travel at the same speed as the source wave These are known as Huygens' wavelets

Answer 38

Young devised the famous double-slit experiment This provided experimental proof that light is a wave that can undergo constructive and destructive interference

Answer 39

Maxwell showed that electric and magnetic fields obeyed the wave equation. This means that light was simply waves made up of electric and magnetic fields travelling perpendicular to one another Later, Maxwell and Hertz discovered the full electromagnetic spectrum

Answer 40

Einstein discovered that light behaves as a particle, as demonstrated by the photoelectric effect He described light in terms of packets of energy called photons Later the scientific community came to understand that light behaves both like a wave and a particle This is known as wave-particle duality

Answer 41

Broad and bright central maximum, twice as wide as side maxima, intensity of maxima decreasing away from centre. outer fringes of the same order are of the same width

Answer 42

* Broad white central maximum, * each subsidiary maxima are composed of a spectrum * subsidiary maxima have violet nearest central maximum and red furthest from centre * The non-central maxima are wider and less intense * dark fringes are smaller (or not present)

Answer 43

The path difference between top and bottom of slit is Δ𝐿 = 𝑠sin⁡𝜃 = 𝑊𝑠/𝐷 First minimum occurs when Δ𝐿=𝜆 So, separation of minima given by 𝑊= 𝜆𝐷 / 𝑠

Answer 44

If the slit was made narrower: * The angle of diffraction is greater * So, the waves spread out more beyond the slit The intensity graph will show that: * The intensity of the maxima decreases * The width of the central maxima increases * The spacing between fringes is wider

Answer 45

A diffraction grating consists of a large number of very thin, equally spaced parallel slits carved into a glass plate at regular spacings distance d apart

Answer 46

Light from each slit undergoes diffraction and overlap Path difference is a whole number of wavelengths Arrive at screen in phase Meet and undergo superposition

Answer 47

The path difference between rays coming from adjacent lines is Δ𝐿=𝑑 sin⁡𝜃 The rays will therefore all be in phase with one another when 𝑑 sin⁡𝜃=𝑛𝜆

Answer 48

𝑑= 10^(−3) / (lines per mm) 𝑑=1/(lines per m)

Answer 49

The largest order that can appear in a diffraction pattern is given by 𝑛_max=𝑑/𝜆 (rounded down) The total number of bright spots that appear in the pattern is given by 𝑁_spots=2𝑛_max+1

Answer 50

Narrow maxima of equal intensity that get further apart at higher orders away from the center

Answer 51

Narrow, white central maximum. A spectrum at 1st order maximum with blue closest to centre and red furthest away. Broader, overlapping spectra at 2nd and higher orders

Answer 52

* Analysing spectra from stars, galaxies, etc. to determine composition, temperature etc. * Measuring the wavelength of a laser * Chemical analysis and identification (e.g absorption spectrum colorimetry)

Answer 53

X-rays are directed at a thin crystal sheet which acts as a diffraction grating to form a diffraction pattern This is because the wavelength of X-rays is similar in size to the gaps between the atoms This diffraction pattern can be used to measure the atomic spacing in certain materials - d in equation

Answer 54

The refractive index 𝑛 of a material is defined to be given by: 𝑛=𝑐/𝑐_𝑠 where 𝑐 is the speed of light in a vacuum 3.00×10^8 m s^(−1) and 𝑐_𝑠 is the speed of light in the material e.g. in glass the speed of light is 2.00×10^8 m s^(−1) so it has a refractive index of 1.50.

Answer 55

When a light ray strikes the boundary between two media, it is partially reflected and partially transmitted The reflected ray has the angle of incidence equal to angle of reflection The refracted ray obeys Snell’s law 𝑛_1 x sin⁡𝜃1 = 𝑛_2 x sin𝜃2 where 𝜃 is angle between ray and normal line

Answer 56

Refraction is: The change in direction of a wave when it passes through a boundary between mediums of different density This change of direction is caused by a change in the speed of different parts of the wavefront as they hit the boundary When a wave refracts it always produces a partial reflection

Answer 57

used to describe a transparent material

Answer 58

The more optically dense the new medium compared to the old one, the slower the waves travel and the smaller the angle of refraction - The light bends towards the normal. The less optically dense the medium, the faster the waves travel and the larger the angle of refraction - The light bends away from the normal. The angles of incidence and refraction are measured from the normal line. This is drawn at 90° to the boundary between the two media

Answer 59

When a wave refracts, its speed and wavelength change, but its frequency remains the same This is noticeable by the fact that the colour of the wave does not change

Answer 60

The wave passes straight through without direction This is because the whole wavefront enters the boundary at the same time

Answer 61

If 𝑛_2 < 𝑛_1 (new medium is less dense than the old one) then there exists a critical angle given by sin𝜃_𝑐 = 𝑛_2 / 𝑛_1 If 𝜃_1 > 𝜃_𝑐, then it is impossible for a refracted ray be transmitted through the boundary, so the light is totally internally reflected. When the angle of incidence = critical angle then: Angle of refraction = 90° The refracted ray is refracted along the boundary between the two materials

Answer 62

An optical fibre is a thin piece of flexible glass, which light can travel down due to total internal reflection.

Answer 63

Communications, such as telephone and internet transmission Medical imaging, such as endoscopes

Answer 64

It propagates the wave/light by TIR as refractive index of core > cladding

Answer 65

* Protect the thin core from damage and scratching * Prevent signal degradation through light escaping the core, which can cause information from the signal to be lost * It keeps the signals secure and maintains the original signal quality * It keeps the core separate from other fibres preventing information crossover

Answer 66

Where the pulses emerging from the fibre are longer than those entering Pulse broadening is caused by modal and material dispersion This can result in the merging of pulses, which distorts the information in the final pulse and increases the amplitude of the signal

Answer 67

When white light is used instead of monochromatic light inside an optical fibre it is separated into all the colours of the spectrum Each wavelength of light travels at the same speed in a vacuum but at different speeds in a medium Violet light has the shortest wavelength, so it travels the slowest in the fibre. This means its angle of incidence on the fibre boundary is smallest compared to the other colours. The angle of incidence is equal to the angle of reflection, so the angle of reflection is also smaller. This means it takes longer for the violet colour to travel down the fibre because it undergoes more reflections

Answer 68

Rays travelling at different angles inside the fibre travel different distances, and so take different times to reach the end of the fibre. This is because each part of the wavefront has a different angle of incidence and consequently a different angle of reflection So each part of the wavefront undergoes total internal reflection a different number of times Hence, each part of the wavefront reaches the end of the fibre at a slightly different time This leads to pulse broadening

Answer 69

Modal dispersion can be reduced by using cladding, which gives a large critical angle, so only rays close to the axis can travel. It can be eliminated entirely by making the fibre narrow enough that it becomes monomodal.

Answer 70

The absorption of a signal in an optical fibre occurs when the fibre absorbs part of the signal’s energy This reduces the amplitude of the signal, which can lead to a loss in the information transmitted

Answer 71

Use an extremely transparent core Use optical fibre repeaters so the pulse is regenerated before significant absorption has taken place

Answer 72

Use a core that is as narrow as possible to reduce the possible differences in the path length of the signal Use of a monochromatic source so the speed of the pulse is constant Use optical fibre repeaters so the pulse is regenerated before significant pulse broadening has taken place Use a single-mode (monomodal) fibre, where only a single wavelength of light passes through the core, to reduce multipath modal dispersion

Answer 73

The path difference between A and B is half a wavelength so A and B are 180° out of phase so destructive intereference occurs.

Answer 74

The equation is only valid if the fringe separation, w, is much less than the distance D from the slits to the screen. If this criteria is not met you will need to use geometry.

Answer 75

The angle of incidence is 55° not 35° Always measure from the normal!

waves Flashcards

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